The present invention relates to telephone key systems, that is, telephone sets which have a number of push buttons or "keys" for selecting one of a plurality of lines to which the key system may be connected. As is conventional, each one of these line keys is provided with a lamp for indicating whether the associated telephone line has been selected.
Telephone key systems or keyset apparatus of the above noted type is well known in the art. Typically, such key systems have served to provide small offices and businesses with convenient and inexpensive telephone systems in cases where each of the office telephones are to be connectable to any one of a plurality of lines. Classically, all telephones in the key system were connected in parallel to all of the incoming telephone lines. While this arrangement is still in use in most installations, it is possible to also provide a central switch or "private branch exchange" at the point where the telephone lines are supplied to the office, and to connect the separate satellite keysets to this switch via individual line chords containing a single transmitting and receiving channel. In this way, the telephone keyset may appear to the user as part of a conventional key system, but it receives information concerning the status of the various telephone lines from the central switch. This type of key system has the advantage over the classical, parallel system that a cable containing each telephone line need not be supplied to each telephone and that the status of each telephone line may be monitored and controlled at a central point to provide cost accounting and other features which would not otherwise be possible.
All key systems, no matter what type, have the common disadvantage of confusing the user as to which line he or she is supposed to be connected. Particularly, when the key system is connected to a large number of lines (say, five or more), it often happens that the user forgets, or is not informed of the line to which his or her particular telephone keyset is, or is to be, connected.
For example, at any particular time, the telephone keyset may have a number of line indicator lamps lighted simultaneously, thus indicating that the indicated lines are in use. If a secretary of the person to whom the call is directed has received a call and placed the caller on "hold", the person to whom the call is directed may not know which line to "pick up" or select. Although, as is conventional, a line which is placed on hold may be so indicated by a flashing lamp, several lines may be on hold simultaneously, so that several lamps are flashing.
As another example, the telephone keyset user may place a party on hold while he or she attends to some other task, such as talking to another party on another line. Thereafter, when he or she wishes to pick up the original line, the number of this line may have been forgotten. As indicated previously, while each line on hold may be indicated by a flashing lamp, there may be several lines on hold at any given time.
As a third example, the key system may be provided with a feature known as "incoming call automatic line preference" which automatically receives calls and rings the appropriate keyset or keysets to which the call is directed. If the user of that keyset is talking on another line, he or she is made aware of the incoming call and can quickly terminate the current conversation. Thereafter, after this station user hangs up the receiver on the telephone keyset hook switch--i.e. goes "on hook" in telephone terminology--the waiting telephone call may be accepted by either pressing the appropriate line key and going "off hook" again or just going off hook and allowing the incoming call automatic line preference to select the line. However, if several lines are in use, and particularly if several indicator lamps are flashing, the station user will know neither which line should be selected, in the case where he or she overrides the incoming call automatic line preference by preselecting the line, nor which line was selected, in the case where he or she goes off hook and lets the incoming call automatic line preference select the line.
Similarly, where the key system is provided with an "outgoing call automatic line preference" and selects an idle line, the last-used line or a prime line, the caller may not know which line he or she has been connected to. This information may be needed by the caller some time later, for example if he or she wishes to place the call on hold and then return to it.